This invention is directed to absorption heat pumps which achieve improved efficiency by widening the difference between the high and low temperatures of the working fluid. In particular, the invention is directed to additives which depress the crystallization/precipitation temperature of salt contained in the working fluid.
In heat pumps of the absorption type, an absorbent, diluted with an absorbed refrigerant, is heated in a generator to vaporize some of the refrigerant. The refrigerant vapor then flows to a condenser where it is condensed to a liquid by heat exchange with an external cooling fluid maintained at a low temperature by a heat sink. The liquefied refrigerant then flows through a valve to an evaporator which vaporizes the refrigerant (usually at low pressure) to produce refrigeration.
The vaporized refrigerant then flows to an absorber where it is absorbed by concentrated absorbent supplied from the generators From the absorber, the diluted absorbent passes to the generator where it is concentrated by heating to vaporize some of the refrigerant, and thus repeat the cycle.
Conventional absorption heat pumps typically employ an aqueous solution of lithium bromide as an absorbent and water as a refrigerant. The operating efficiency of these heat pumps increases with the difference between the highest fluid temperature where the solution is dilute in lithium bromide and water is being vaporized, and the lowest fluid temperature where the solution is very concentrated in lithium bromide and water is being absorbed. When operating in a refrigeration/air conditioning mode, the high temperature is fixed by the ambient temperature. When operating in a temperature boosting mode, the high temperatures can reach 98xc2x0 C., 177xc2x0 C., and 232xc2x0 C. for single, double and triple effect machines, respectively. Some of these machines are described in Herold and Radermacher xe2x80x9cAbsorption Heat Pumpsxe2x80x9d, Mechanical Engineering, August 1989, pp.68-73. Since the high cycle temperature is generally fixed by the application and/or pump type, the efficiency of the cycle can be increased by lowering the low cycle temperature.
As the low cycle temperature is reduced in an air conditioning application, the concentration of lithium bromide must be increased in order to permit the continued absorption of water vapor. As the salt concentration is increased and the temperature is decreased, a solubility limit is approached. If the solubility limit of lithium bromide in water is exceeded, hydrated salt crystals may form which block the flow circulation in the absorber, rendering it useless. Thus, conventional absorption heat pumps use solutions containing about 60-62% salt, and operate at a minimum fluid temperature of about 4-7xc2x0 C. in air conditioning applications. For heating applications, the salt concentration may be lowered, to prevent freezing of the solution at temperatures down to xe2x88x9225xc2x0 C. or lower.
Absorption heat pumps have many large-scale uses in industrial air-conditioning and refrigeration, as well as heating and temperature boosting. There is always a need or desire for more efficient heat pumps which maximize the difference between the high and low fluid temperatures at different parts of the cycle.
The present invention is an absorption heat pump which achieves a greater difference between the high and low fluid temperatures of the circulation fluid. by reducing the minimum fluid temperature to levels not previously contemplated. Additives have been discovered which inhibit the crystallization and precipitation of lithium bromide from water at concentrations of 60-62% lithium bromide and temperatures below about 4xc2x0 C. without adversely affecting 1) the heat capacity of the solution, 2) the solution rheological properties, 3) the solution diffusion or mass transfer coefficients, or 4) the ability of the solution to absorb water vapor and transfer heat in the process. In an absorption cycle, these additives permit operation at a lower low temperature, thereby improving the efficiency of the cycle. The increased efficiency makes the absorption heat pump more cost effective compared to conventional refrigeration technologies.
The additives for the aqueous lithium bromide solution can reduce the minimum low fluid temperature from about 4-7xc2x0 C. to about 0xc2x0 C. or lower. Some of the additives can reduce the minimum low fluid temperature to xe2x88x925xc2x0 C. or lower, to xe2x88x928xc2x0 C. or lower, or even to xe2x88x9210xc2x0 C. or lower. The additives can also be used to reduce the minimum low fluid temperature in applications using lower concentrations of lithium bromide in water.
Suitable additives are those which form complexes with lithium and/or bromine ions in aqueous solution. The additives and complexes formed may 1) decrease the crystallization driving force, causing supersaturation, 2) increase the critical supersaturation needed for effective nucleation, and/or 3) decrease the crystal growth rate. Useful additives include compounds which form complexes with the lithium and bromine ions in solution, and which alter the surface energy of crystal embryos formed in solution just prior to nucleation.
With the foregoing in mind, it is a feature and advantage of the invention to provide an aqueous lithium bromide solution useful in absorption heat pumps, which has a wider range of cycle temperature due to a lower minimum temperature for the onset of crystallization.
It is also a feature and advantage of the invention to provide an absorption heat pump having greater efficiency due to a wider range of cycle temperatures and a lower minimum cycle temperature.